After semiconductors are manufactured, they are subjected to chemical-mechanical polishing (CMP). The most common way to clean silicon wafers after chemical mechanical polishing is with a brush cleaning tool. The problem with such as system is that the abrasiveness of the brush often damages the wafer. Alternatively, there are some systems where spray tools are employed to clean wafer surfaces. These methods do not have optimal cleaning ability. Another technique is to use megasonics combined with immersion into other solvents. This technique, and the chemicals used is described in detail below.
Megasonics
Megasonics is a cleaning method for contamination-sensitive products. Integrated circuit, hard drive, raw silicon, mask, flat panel display, and other devices affected by contamination are cleaned using megasonics to help meet stringent cleanliness requirements. The megasonic method is commonly employed to clean semiconductor wafers themselves. A megasonic cleaning system uses high frequency (700-1,000 KHz) acoustic energy pressure waves produced in a liquid. The liquid is contained in a cleaning tank. Because the pressure waves are effective for removing particles from silicon wafers without causing damage to the wafers themselves, it is an excellent nondestructive cleaning technique. Megasonics maybe used to effectively removing 0.15-micron particles from silicon wafers and other products, without damage.
Megasonics cleaning uses the piezoelectric effect (the generation of electric polarization in certain dielectric crystals as a result of the application of mechanical stress) to enable removal of submicron particles from substrates. A ceramic piezoelectric crystal is excited by high-frequency AC voltage, causing it to vibrate. This vibration generates an acoustic wave that is transmitted to and through the cleaning fluid, producing controlled cavitation. Cavitation is the formation of gas bubbles within the liquid created by the application of mechanical force. Because the cavitation is formed by acoustic waves, it is said that megasonic employs xe2x80x9cacoustic cavitation.xe2x80x9d Acoustic cavitation is caused by the pressure variations in sound waves moving through the cleaning fluid. As the acoustic wave passes across the surface of the wafer, particles are removed. Acoustic cavitation provides sufficient energy to overcome particle adhesion forces and cause particles to be removed. Controlled megasonics cavitation becomes acoustic streaming, which pushes the particles away so they do not reattach to the material being cleaned.
It is useful to contrast megasonic cleaning with ultrasonic cleaning to fully understand the megasonic process. The fundamental difference between ultrasonic cleaning and megasonics cleaning lies in the frequency that is used to generate the acoustic waves. Ultrasonic cleaning uses frequencies between 20-350 kH. These frequencies produce random cavitation. Megasonics cleaning uses higher frequencies at 700-1000 kHz. This produces controlled cavitation.
An important distinction between the two methods is that the higher megasonic frequencies do not cause the violent cavitation effects found with ultrasonic frequencies. This significantly reduces or eliminates cavitation erosion and the likelihood of surface damage to the product being cleaned. Parts that would be damaged by ultrasonic frequencies or cavitation effects can often be cleaned without damage in a megasonic bath using the same solution. With ultrasonics, cavitation occurs throughout the tank, and all sides of submerged parts are cleaned. With megasonics, only the side of the part that is facing the piezo electric device is cleaned.
Chemicals Used in Megasonics
Megasonics cleaning may be used with a variety of chemistries. Although megasonics is used primarily for particle removal, it may also be used to increase the efficiency of chemical cleaning with surfactants or detergents. Removal of other contaminants depends on the solutions in the tank.
Deionized water (DI) water is commonly used in megasonic application. An xe2x80x9cionxe2x80x9d is an atom or group of atoms that is not electrically neutral, but instead carries a positive or negative electric charge. As the names suggest, deionized water is water without such ions present. Deionized Water of ultrahigh purity (very low in contaminants) is of high electrical resistance which makes it useful for cleaning various semiconductor substrates.
Wafers are often rinsed in pure DI water after being immersed in a chemical bath both to remove the chemicals, and to further clean the wafer. As relates to megasonics, the chemicals chosen are substances (aqueous or organic) designed to clean the semiconductor wafer (or other assembly) by dissolving the contaminants present on its surface. The rinse of the DI water removes the residue of chemical that still remains on the wafer from previous immersions. DI water has a pH of 7 meaning that it neither an alkaline nor acidic but rather is neutral.
DI water may also be used to dilute the concentration of a chemical in the megasonic bath (or other immersion baths). It is common to mix various chemicals in different concentrations in DI water. One chemical commonly used is ammonia hydroxide. Another is hydrogen peroxide. Hydrogen peroxide is a strong oxidizing chemical used in particle removal chemistries. SC-1, a solution commonly used in megasonics, is a mixture of ammonia hydroxide, hydrogen peroxide and water. Hydrofluoric acid (HF) is also commonly used. Hydrofluoric acid is a corrosive acid used in etching and cleaning. It is commonly used for the removal of metals or thin oxides of glass. Another common corrosive acid used in particle removal chemistries is hydrochloric acid (HCL). Hydrochloric acid is commonly found in SC-2 or HPM solution which is a solution used for metallic removal.
Surfactants may also be used in megasonics. A surfactant is a surface active solution, and is used to lower the surface tension of a liquid. Surfactants reduce interfacial tension between two liquids or a liquid and a solid. Because of this property, surfactants reduce the ability for particles to adhere to the wafer. Detergents are made up principally from surfactants. One surfactant commonly used to clean wafers is known as NWC-601.
It is generally known that exposure time, the length of time the wafer is subjected to the megasonic cleaning process, is an exceptionally important variable affecting megasonics cleaning. Indeed, it is well-known that as exposure time increases, particle redeposition decreases. The inherent problem with using increased exposure times is that cycle time (the time needed to clean each batch of wafers) correspondingly increases, thereby adding additional manufacturing cost and production time. Today, typical exposure times are 10 to 20 minutes. The exposure times of 7 minutes or less are not currently employed for post-chemical mechanical polish or back-grinding operations, and are commonly believed to be insufficient to reach desired levels of cleaning efficiency. What is not known is that using two or more megasonic cycles of shorter duration, as opposed to a single cycle of longer duration, increases cleaning efficiency (the ratio of particles on a material before the cleaning process to the particles remaining after cleaning) without significantly effecting the overall cycle time.
The term xe2x80x9ccyclexe2x80x9d has a specific meaning in the semiconductor fabrication industry. By xe2x80x9cchemical cyclexe2x80x9d I mean exposing the wafer to the same, or approximately the same, chemicals in close temporal proximity to achieve a desired result. One aspect of my invention is performing one xe2x80x9cchemical cyclexe2x80x9d as two separate megasonic chemical xe2x80x9csub-cycles.xe2x80x9d Preferably, the very chemical bath of the first chemical megasonic sub-cycle is used again for the second chemical megasonic sub-cycle, and there is a rinse in between. Where the megasonic chemical bath is not a sub-cycle, it is simply labeled a xe2x80x9cchemical megasonic cyclexe2x80x9d instead of a xe2x80x9cchemical megasonic sub-cycle.xe2x80x9d Thus, I have used the terms xe2x80x9cfirst chemical megasonic sub-cyclexe2x80x9d and xe2x80x9csecond megasonic sub-cyclexe2x80x9d because they form a single chemical megasonic cycle. However, I have used the term xe2x80x9cthird chemical megasonic cyclexe2x80x9d and xe2x80x9cfourth chemical megasonic cyclexe2x80x9d because they are not sub-cycles and constitute complete individual chemical cycles by themselves.
There are other advantages to shortened exposure times. For instance, shorter exposure times allows the same chemicals to be used for a larger number of lots. Also, since the wafers are rinsed more frequently, the chemical baths themselves retain less contamination.
The main forces adhering particles to the surface of the wafers are Van der Waals forces (weak forces), electrostatic forces, and chemical bonds. (Van der Waals forces are weak attractive forces between two atoms or non-polar molecules, which arises because a fluctuating dipole moment in one molecule induces a dipole moment in another molecule, and then the two dipole moments interacting.) Another discovery regards the matching of the pH level of the solutions to be used to the pH of the particles to be removed from the surface. The pH adjustment improves the efficiency of breaking the electrostatic forces holding the contamination to the surface of the substance to be cleaned. The current practice is to use SC-1 solution at a standard ratio of 5 parts DI water, 1 part ammonia hydroxide, and 1 part hydrogen peroxide.
Previously, it had been thought that it was not desirable to match the pH of the chemical solution to the substance being removed from the wafer. For instance, it was thought that if the substance to be removed is alkaline (pH over 7 to 14) that the chemical should be acidic (pH from 0 to under 7), and vice versa. To the contrary, I discovered that matching the pH of the chemical solution to the pH of the substance to be removed is desirable because it increases cleaning efficiency. When the pH level of the chemical is matched to the pH level of the substance being removed, the chemical is not trying to dissolve the substance to be removed. Nor is the chemical trying to change the solution to be removed into a salt or anything other substance. (It is commonly known that the admixture of an alkaline substance to an acid produces salt and water.) This quality makes immersion into the cleaning solution easier. For instance, the formation of salt has been shown to be particularly pernicious to the cleaning process.
In my invention, a multi-pass wafer cleaning process employs immersions baths, megasonic units, and various chemistries to clean the slurry from the wafers and leave the wafers low in particles and metals. The act of subjecting the wafer to a chemical in the megasonic bath is termed a xe2x80x9cchemical megasonic sub-cycle cycle.xe2x80x9d Because the invention contemplates more than one chemical megasonic sub-cycle, I have employed the term xe2x80x9cfirst megasonic cycle,xe2x80x9d xe2x80x9csecond megasonic cycle,xe2x80x9d etcetera to indicate each one. The term xe2x80x9cfirstxe2x80x9d however is not to be construed as first in order. The periods of time for the first megasonic time being 20 minutes, 17 minutes, 14 minutes, 11 minutes, 9 minutes, 6 minutes, 4 minutes, 2 minutes, 1 minute are contemplated in my invention.
A slurry is a free flowing suspension of fine solid material in liquid. Slurries are semifluid substances, which usually are a mixture of abrasive compounds in an aqueous solution. The slurry is intentionally added during the chemical mechanical polishing (CMP) stage to smooth the wafers. The invention is facilitated by the discovery that during the initial (first to be performed) chemical megasonic sub-cycle, it is best to match the pH of the chemical, preferably SC-1, to the pH of the slurry to be removed.
The invention also employs a number of megasonic cycles whereby the wafer is immersed in a megasonic bath of pure DI water. These are termed xe2x80x9cDI water rinse cycles.xe2x80x9d Again, the terms xe2x80x9cfirst DI water rinse cycle,xe2x80x9d xe2x80x9csecond DI water rinse cyclexe2x80x9d etcetera, are employed. While the invention could be performed without the use of the DI water megasonic rinsing, the cleaning efficiency improves dramatically with such rinsing. This allows one batch of semiconductors to be in the megasonic unit while another batch is being rinsed. In the preferred embodiment, all the steps of the method are continuously performed such that the wafers do not dry until the process is complete.
Some of the rinse cycles may be performed using a quick dump rinser. In a dump rinser, wafers are placed into a dry rinser and immediately sprayed with DI water. While they are being sprayed, the cavity of the rinser is rapidly filled with water. As the water over overflows the top, a trap door in the bottom swings open and the water is dumped instantly into the drain system. This fill-and-dump action is repeated several times until the wafers are entirely mixed. Dump rinsing is favored because all of the rinsing takes place in one cavity, which saves equipment and space. It is also a system that can be automated, so that he operation needs to load the wafers in (this can be done automatically) and push a button.